Our invention relates to generally an apparatus for the transfer (reading and/or writing) of data with disklike record media such as magnetic disks notably including those of the "fixed" or hard disk variety. More particularly, our invention pertains to a closed-loop servo system for the positional control of a data transducer in such apparatus in seeking any desired one of a plurality or multiplicity of concentric data tracks on a disklike record medium. Our invention also particularly concerns the record medium itself, featuring code patterns prerecorded thereon for identification of the individual tracks.
We are aware of three typical methods have been known and used for positioning the transducer with respect to a multiplicity of concentric data tracks on hard magnetic disks. They are: (a) the servo system with a servo head and a replicated servo surface on the disk; (b) the servo system employing an encoder for the detection of the positional relationship between the head and the data tracks; and (c) the open-loop system with a stepper motor for moving the head a distance specified by the number of stepping pulses.
The first recited servo positioning system has the disadvantage that only one side of the disk can be utilized for data storage, the other side being used exclusively for servo control positioning of the transducer. Another drawback is the high cost of the equipment needed for creating the servo control information on the disk. The second mentioned servo system is also objectionable because of the high cost of the equipment including the encoder which is complex in construction. The third, open-loop system is unsatisfactory in the speed of seek operation.
It has also been suggested to prerecord addresses at individual tracks on the disk. In seeking any of such addressed tracks the transducer reads the prerecorded addresses until it is positioned on the destination track. Although capable of accurately positioning the transducer on any desired track, this scheme has the inherent weakness that the track addresses demand a greater number of bits for identification of individual tracks as the track density per disk is increased, as is the current trend. Consequently, with such greater track density disks, it becomes increasingly difficult to move the transducer across the closely spaced tracks as sufficiently high speed as the transducer must read the addresses comprised of greater numbers of bits.